1. Field of the Invention:
The present invention relates to printing devices, and more particularly, to a novel stencil screen and to methods and means for its manufacture and use.
2. Description of the Prior Art:
In screen printing, a stencil is prepared on a porous base having mesh openings of a size sufficient to pass liquid or solid toner particles to a surface to be copied. The non-image areas of the screen are blocked with a suitable ink-impervious material in a pattern corresponding to a negative of the open image printing pattern. Ink is applied to the screen and is directly transferred to the substrate through the porous base or may be accelerated to the substrate by electrostatic forces. The use of a metal screen in the case of an electrostatic printing process, such as described in U.S. Pat. No. 3,081,698, Childress et al, enables one to utilize the stencil screen as one electrode of the electrostatic field for accelerating the toner to the substrate.
There are three principal methods of commercially preparing the stencil. By far, the most common and popular technique is the so-called "direct process" in which a light-sensitized hydrocolloid, for example, gelatin or polyvinyl alcohol, is coated directly onto the mesh by means of a reservoir-type coater. After the coating is dried, a second coating is applied in a similar manner to avoid pinholes. Though the coating is simple to apply, it is rather slow-drying, and the use of higher temperatures can cause bubbling or premature hardening of the colloid.
The stencil is prepared by exposing the colloid selectively to an arc lamp which tans and hardens the illuminated areas. The untanned material is removed by washing with water jets at a carefully controlled temperature. This operation is delicate and requires an experienced operator. The screen must be again dried before it is ready for use. The direct process screen has the advantage that the stencil is exposed and developed after the coating is applied to the screen. However, the coating emulsion fills the spaces between the filaments which tend to adjust to a location midway between the front and rear surface. Therefore, the quality of resolution obtainable is limited.
The second process is the so-called "transfer method" in which a presensitized film coating, such as a bichromated polyvinyl alcohol, is similarly exposed and developed while on a support. The residual emulsion is hardened in acetic acid and the developed coating is then transferred to the screen and allowed to dry. Presumably, the coating is somewhat swollen and contracts and adheres to the filaments of the screen. The adhesion to the screen is quite poor, and consequently, multifilament threads, such as silk, are usually utilized. Silk does not perform very well as an electrostatic stencil since the pores between the filaments tend to become clogged quite easily. Again, an extremely skilled operator is required in the transfer method, and it is very easy to wasy away too much of the emulsion in the development step. In fact, whole chunks may break away, destroying the stencil coating. An even more serious problem is the fact that the dimensional tolerances are very difficult to maintain during transfer of the developed stencil to the screen which results in distortions of the image.
The third process is not photographic and is confined to drawing images directly on the stencil and cutting them out or drawing them in a material that is overcoated and leached out. This is called the "Tusche method" and is confined primarily to artists and other highly skilled craftsmen and is utilized to achieve unusual effects by screen printing techniques. The Tusche method has the advantage that a cut-out stencil can be prepared without the requirement of any chemical developing steps. It, of course, suffers a disadvantage that copy cannot be formed as a stencil by direct application of light to the stencil blank.
Thermal reactions have been suggested as a means of forming patterns in stencil surfaces for reproduction by mimeograph or other related techniques. However, the resolution of these thermal images is extremely low with conventional heating sources. The coatings are of a soft and waxy nature and are not adaptable for printing by either electrostatic screen printing or conventional silk screen processes, and furthermore, the substrates are typically formed of randomly oriented fibrous material, such as paper. The openings formed through paper backings are of a non-uniform dimension and, therefore, wet and dry ink particles are not uniformly passed through the backing.
Furthermore, in the silk screen and electrostatic screen processes, it is usually necessary to stretch the screen onto a frame to provide dimensional stability to the image or to bend the frame into an arcuate or curvilinear shape to print on correspondingly shaped objects. The tension in silk screen printing is sufficient to cause the screen to snap back after deflection in wet ink printing. In electrostatic printing, the screen is stretched with at least sufficient tension to resist deflection by the D.C. electrostatic field. This requires the use of a resilient support having a high tensile strength which is not the case with the wide-mesh paper-back stencils utilized in the mimeograph and related arts.